1. Field of the Invention
The present invention concerns a photometer for use in a camera and, more specifically, it relates to a photometer for use in a camera that measures the brightness at the portions corresponding to the central area and the marginal area on a picture plane of a camera.
2. Discussion of Background
It is desired in a camera having the function for automatic exposure control that an adequate exposure control can be conducted, for example, to objects in the normal light state, as well as to objects having highlight and shadow portions such as objects in back-light state. This requires exact light metering, that is, exact metering of the brightness of the object. However, the accuracy of the metering varies greatly depending on the way of measuring the light from the respective areas of an object, that is, depending on the arrangement of a pattern having sensitivity on a picture plane of a camera. The metering method can be classified into partial metering, average metering, center weighted metering, etc. depending on the portions to be measured on a picture plane of a camera. In the case of an object in the back-light state described above, it has been considered effective to conduct metering for the marginal area and the central area on the picture plane. Therefore, there has been used a photometer having two photoreceiving elements, that is, a photoreceiving element for metering the marginal area and a photoreceiving element for metering the central area to detect that an object is in the back-light state. However, the photoconductive material of the photoreceiving element is generally made of such semiconductor as cadmium sulfide (CdS), cadmium selenide (CdSe), etc. Therefore, it has been difficult to keep the characteristics of respective photoreceiving elements within a narrow allowable range. Accordingly, a compensating circuit, etc. has been disposed additionally for absorbing such scattering of the characteristics of the photoreceiving elements for use in marginal and central area. This increases the material cost and increases the number of fabrication and adjustment steps, which results in the increase of the production cost.
Referring, for instance, to transistors which are typical semiconductor products, since it is not practical to keep their characteristics within a narrow range as described above, it has been a general practice to allow the scattering of the characteristics and classify them into several groups depending on the degree of scattering upon marketing them. Those goods referred to as pair transistors incidentally having characteristics substantially identical with each other selected on the side of manufactures, are supplied on the market as paired products. Since such pairs are selected by chance, yield of the products is low and the cost is naturally high. The situation is similar in the case of photoreceiving elements and it is apparently disadvantageous in view of the cost to select pairs of photoreceiving elements having substantially identical characteristics in the mass products such as cameras.
Therefore, the advent of photoreceiving elements of uniform characteristics capable of conducting photo-metering for the marginal area and the central area on the picture plane has been required.
In addition to that, to detect the back-light state of an object in the exposure control section, complicated analog or digital calculations are necessary for compensating the output from each of the photoreceiving elements for photo-metering the marginal and central areas, consequently circuits and devices for the calculations are necessary which obstruct the simplification of the structure.